On a navigation system, contents of roads and display elements such as facilities to be displayed on a map are decided for each map scale (scale of a map). As for roads, for example, a large-scale wide-area map displays main roads such as expressways and national highways, and as the map scale reduces, more narrow roads come to be displayed. Accordingly, when the map scale is switched, the display elements being displayed can sometimes disappear from the screen suddenly, thereby confusing a user.
On the other hand, a technique is known which displays, without losing the continuity of roads, a map in a wide area in all directions placing a current position at the center. According to the technique, the map is displayed in such a manner that as the distance from the current position assigned as the origin increases, the scale is increased continuously in circumference directions (see Patent Document 1, for example).    Patent Document 1: Japanese Patent Laid-Open No. 9-44085/1997.
However, according to the technique disclosed in the foregoing Patent Document 1, since the display elements are displayed without losing continuity in the same manner as a fisheye lens, the display elements with different scales are mixedly displayed on a single screen, which presents a problem of distorting the image.
In addition, it is necessary to calculate the display positions of all the display elements like roads using a converting expression with high computation cost, which presents a problem of making it difficult for limited hardware resources the built-in equipment such as a car navigation system has to perform smooth scrolling.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide a navigation system and a display method of a road network on the same system aiming at achieving display without losing continuity of display elements such as roads and facilities without distortion, and at reducing a load of the hardware resources at the time of calculating the display positions.